A technique called the “MS/MS analysis (tandem analysis)” is known as a mass spectrometric technique used for identifying or quantifying a target component contained in a sample. For example, the MS/MS analysis is performed using a mass spectrometer (such as a tandem quadrupole mass spectrometer) including: a front mass separator section for selecting a precursor ion; a fragmenting section, such as a collision cell, for fragmenting the precursor ion into product ions; and a rear mass separator section for selecting a product ion.
An MRM measurement is one mode of the measurement in the MS/MS analysis. In the MRM measurement, the mass-to-charge ratio at which an ion is allowed to pass through is fixed in each of the front and rear mass separator sections to measure the intensity of a specific product ion for a specific precursor ion. Such a combination of the precursor ion and product ion is called the “MRM transition”. In the MRM measurement, the front and rear mass separator sections remove ions originating from compounds which are not measurement targets, ions originating from foreign substances, as well as neutral particles which have not been ionized. Therefore, ion intensity signals with a high signal-to-noise (SN) ratio can be obtained.
Due to such merits, the MRM measurement has been used for an analysis of a sample containing a plurality of target compounds, such as a sample collected from soil or sample of biological origin. For an analysis of a sample containing a plurality of target compounds, a chromatograph mass spectrometer which includes a chromatogram (gas chromatograph or liquid chromatograph) combined with a mass spectrometer having the previously described configuration is used. The plurality of target components contained in the sample are temporally separated from each other by a column in the chromatograph and introduced into the mass spectrometer, to be individually subjected to an MRM measurement.
In an MRM measurement in a chromatograph mass spectrometer, an analysis operator determines the contents (method) of a series of measurements on mass spectrometry software by entering, for each of the target compounds, one or more MRM transitions which correspond to the target compound as well as a time segment during which an MRM measurement using each of those MRM transitions is executed. The analysis operator prepares a measurement execution file (method file) in which the entered contents are described. The analysis operator also gives a name for identifying the measurement condition (event name) to each individual measurement condition (i.e. the combination of an MRM transition and an execution time). FIG. 1 shows an example of the method in the case of performing a measurement for 50 kinds of target compounds using two MRM transitions for each compound.
The form of fragmentation of a precursor ion varies depending on the magnitude of the fragmentation energy. In the method file mentioned earlier, the magnitude of the ion fragmentation energy in the fragmenting section is set at a previously determined value (preset value); the fragmentation energy is not set for each of the set MRM transitions at a value which yields the highest sensitivity for the detection of the product ion. Therefore, it is necessary to optimize the magnitude of the fragmentation energy for each MRM transition in order to perform a measurement of each target compound with high sensitivity (for example, see Patent Literature 1 or 2). In the case where the fragmenting section is a collision cell, the fragmentation energy is normally called the “collision energy (CE)”.
If a standard sample of the target component in the form of a pure substance is available, the CE value which yields the highest sensitivity for the detection of the product ion can be determined by directly introducing the standard sample into the mass spectrometer and sequentially changing the CE value. However, it is difficult to obtain a standard sample for a target compound contained in a sample collected from soil or a sample of biological origin. In such a case, the CE value is optimized by a procedure as shown in FIG. 3 using a chromatograph mass spectrometer.
Initially, a method file prepared on the mass spectrometry software in the previously described manner (this file is hereinafter called the “parent method file”) is exported to a file in a specified format, such as the csv format (Step S101). This file is subsequently read by an appropriate software application, such as a spreadsheet. The method described in the parent method file (this method is hereinafter called the “parent method”) includes measurements to be performed using the MRM transitions individually (these measurements are hereinafter called “parent events”). Those measurements are divided into groups (Step S102), and a method which corresponds to one group (this method is hereinafter called the “child method”) is prepared for each group (Step S103).
Subsequently, in each of the child methods, a plurality of different CE candidate values are set for each MRM transition (Step S104), and a plurality of events which respectively correspond to the combinations of the MRM transition and CE candidate values (these events are hereinafter called “child events”) are created. The child methods which have been updated through the creation of the child events are saved to a file in an appropriate format, e.g. csv, and then imported from the file into the mass spectrometry software to create a child method file (Step S105). FIG. 2 shows an example of the child events created from the method shown in FIG. 1. All parent events are divided into groups of ten parent events. Ten child methods corresponding to those groups are created. In each child method, a total of 12 child events are created for each MRM transition, with the CE candidate value set at 12 levels ranging from 5V to 60V at intervals of 5V.
While the sample is introduced into the chromatograph, one of the child methods is executed, and the intensity of the product ion is measured at each of the different CE candidate values for each MRM transition described in the child method (Step S106). Subsequently, for each MRM transition, the analysis operator checks the measured result and selects one CE candidate value at which the product ion has been detected with the highest sensitivity. This CE candidate value is determined as the CE value for the MRM transition concerned (Step S107).
After all child method files have been executed and the CE values for all MRM transitions have been determined, those CE values are written in the parent method file, and this file is updated (Step S108).
In the conventional method described thus far, the analysis operator needs to manually perform the tasks of exporting the parent method file, dividing the parent method (creation of child methods), entering the plurality of CE values (creation of child events), updating the child methods, and importing the updated data into the mass spectrometry software (creation of child method files). Those tasks require a considerable amount of time and labor.
To reduce the amount of work by the analysis operator, software applications have been proposed which can automatically create child method files by extracting parent methods in batches of a predetermined number from a parent method file in the described order (for example, see Non Patent Literature 1).